I happened across the answer to this question the last time I visited the A&U archive.

The Allen & Unwin and Houghton Mifflin copies were printed and bound at the same time in the U.K. The only difference between the two editions is that the Houghton Mifflin copies have the price on the front flap of the dj in dollars rather than shillings.

Due to a mistake by the binder, 2,000 A&U copies were sent to the U.S. instead of 2,000 HM copies. The cheapest solution was to re-price each dj. I guess A&U opted to use stickers rather than clip them.

If you could supply a photo or scan that would be great.

I think that the impression number on the dj is a slip by A&U - 3rd Impression copies usually come in a 4th Impression dj, and 4th Impression copies come in a 5th Impression dj.